1. Field of the Invention
The present invention relates generally to a printing apparatus capable of forming images by overlappingly performing print processes for a plurality of colors, and more particular to a printing apparatus making use of an aqueous ink and a non-aqueous ink.
2. Description of the Background Art
Conventionally, there are known printing apparatuses capable of forming an image by overlappingly performing print processes of discharging inks of different colors from ink heads.
This type of printing apparatus is provided with cartridges in which standard and particular color inks are contained. The standard color inks (for example, Y: yellow, M: magenta; C: cyan) are aqueous or water-based inks containing a water-soluble solvent. The particular color ink (for example, K: black) is a non-aqueous or oil-based ink containing a water-insoluble solvent or an aqueous or water-based ink. For example, Japanese Patent Published Application No. Hei 11-165420 discloses an example of this technique.
However, in the case of the technique described in the above publication, an aqueous ink is used as a standard color ink, and a non-aqueous ink or aqueous ink is used as a particular color ink. Because of this, depending upon the print coverages of the respective color inks, the image on the printed side can be seen through from the backside (referred to as see-through phenomenon), and that paper becomes likely to be jammed in the paper path.
More specifically speaking, for example, in the case where black is printed with a non-aqueous ink, the pigment is hard to be fixed to the surface of the print sheet such that the density of black cannot be increased in a solid black area (which is filled in with black ink alone). If much ink is used to increase the density, the image on the printed side becomes seen through from the back side.
On the other hand, in the case where all the standard color inks are aqueous inks, when the print coverage of the standard color inks is high, a substantial amount of water contained in the ink tends to deform the print sheet and easily result in paper jam, and thereby there is a problem that it is difficult to increase the printing speed.